1. Field of the Invention
The present invention relates generally to a variable damping force shock absorber with a rotary valve for adjusting flow restriction of a working fluid for adjusting damping force to be generated in response to input shock. More specifically, the invention relates to a rotary actuator for driving the rotary valve for rotatingly drive the latter in order to adjust flow restriction against fluid flow in order to adjust damping characteristics.
2. Description of the Background Art
In the recent years, there have been developed various constructions of variable damping force shock absorber assemblies which is variable of damping characteristics for facilitating variable suspension characteristics in an automotive suspension system. Among such various constructions of variable damping force shock absorber assemblies, some of the shock absorber assemblies includes rotary valves to be rotatingly driven for varying flow resistance against working fluid in the shock absorbers and whereby adjusting damping characteristics. Such rotary valve-type variable damping force shock absorber assemblies have been disclosed in the U.S. Pat. No. 4,600,215, issued on Jul. 15, 1986, to Kuroki et al, for example. In the shown construction, the rotary valve member defines a plurality of orifices respectively having different path areas for varying flow rate of the working fluid to flow between upper and lower fluid chambers in the shock absorber. The rotary valve member, as driven, varies angular position to establish fluid communication between the aforementioned upper and lower fluid chambers of the shock absorber through one of the orifices. With this construction, flow restriction magnitude for the working fluid is variable depending upon the angular position of the rotary valve member for varying the damping characteristics of the variable damping force shock absorber assembly.
On the other hand, in order to drive the rotary valve member for adjusting the damping characteristics, an electromagnetically operable actuator may be provided in the shock absorber assembly. One of the examples of such electromagnetically operable actuator has been disclosed in the Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 58-72546. The actuator disclosed in the above-identified Japanese Utility Model First Publication, comprises a stationary table fixed onto the top of a piston rod, an actuation rod drivingly connected to the rotary valve member, a rotor having permanent magnets and fixed to the actuation rod, and a stator which has a plurality of electromagnets. The electromagnets are arranged at positions radial outside of the permanent magnet and designed to be selectively energized for driving the rotor.
In order to improve such prior proposed construction of the variable damping force shock absorber, U.S. Pat. No. 4,776,437, issued on Oct. 11, 1988, which has been assigned to the common assignee to the present invention. The proposed variable damping force shock absorber includes an improved rotary actuator which is designed for rotatingly drive a rotary body, such as a rotary valve member for adjusting damping force to be created by a variable damping force shock absorber. The rotary actuator takes a layout of a permanent magnet and an electromagnet arranged in vertically spaced relationship. The vertical layout of the permanent magnet and the electromagnet reduces the plane area required for installation of the actuator.
In the shown construction, the rotary actuator comprises a housing, an output shaft, a rotor and a stator. The housing is made of magnetically conductive metallic material for serving as part of a magnetic circuit, and includes an upper cover member and an essentially disc-shaped base plate. The upper cover member is of generally reversed bowl-shaped configuration having axially extending circumferential wall section and laterally extending upper wall section adjoining at the circumferential edge to the circumferential wall section. The lower edge of the circumferential section of the upper cover member engages with the circumferential edge of the base plate in order to define an enclosed space therebetween. The output shaft, the rotor and the stator are housed within the enclosed space. The flanges are integrally formed with the base plate.
The metallic housing in the prior proposed construction encounters a difficulty in assembling. Namely, the upper cover member and the base plate are assembled to each other by calking welding or bonding. When welding is used for assembling the upper cover member and the base plate, paint can be removed during welding operation. This requires additional rust proving treatment after welding operation. Furthermore, air tight seal is required for assuring air tight engagement between the cover member and the base plate. On the other hand, when bonding is used for assembling the cover member and the base plate, movable parts which have to be movably assembled can be fixed by bonding agent which flow out from the bonding section. This lowers the yield of the product.